Maths Olympiad Prep

Library / /10 of 41

Number theory Difficulty 5.0 AIME, harder Prove it New Zealand

Problem:

Show that there are infinitely many triples (a,b,c)(a, b, c) of positive integers such that

a2+b2+c2+(a+b+c)2=abc.a^{2} + b^{2} + c^{2} + (a + b + c)^{2} = abc.

Solution

Solution:

Note that a=b=c=12a = b = c = 12 is a solution. Now, fix c=12c = 12, the original equation becomes

{a2+b2+144+(a+b+12)2=12ab}{2a2+(2410b)a+(2b2+24b+288)=0}(1) \begin{array}{c} \{a^{2} + b^{2} + 144 + (a + b + 12)^{2} = 12ab\} \\ \{\Rightarrow 2a^{2} + (24 - 10b)a + (2b^{2} + 24b + 288) = 0\} \end{array} \quad (1)

We see that by Vieta's theorem, taking (1) as a polynomial in aa, there are 2 solutions of aa, adding up to 5b125b - 12.

Thus, if aba \leq b then we have a new set of solutions: (5b12a,b,12)(5b - 12 - a, b, 12), which by symmetry gives (b,5b12a,12)(b, 5b - 12 - a, 12) as a bigger set of solutions in a,ba, b (non-strict in aa, strict in bb as 5b12a5bbb=3b>b5b - 12 - a \geq 5b - b - b = 3b > b) when a,b12a, b \geq 12.

Since we can repeat this process infinitely, as a,ba, b increases they always fulfill the requirement that a,b12a, b \geq 12, thus we can generate infinitely many solutions.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.